The invention relates to an improved method and arrangement for cell reselection by a mobile station in a cellular radio system. In particular the invention relates to cell reselection in a situation where the mobile station is attached to a special service such as the General Packet Radio Service (GPRS), for example, offered by the current cell.
To provide background for the invention it will be next described by way of example prior-art arrangements for changing the active cell in the GSM (Global System for Mobile telecommunications) system and in a GPRS service used in the GSM.
FIG. 1 shows the basic structure of digital cellular systems. Typically, a mobile communications network comprises at least one core network CN and one or more radio access networks RAN. The core networks CN consist of various central systems which may offer various intelligent network services in addition to versatile communications possibilities. A core network comprises mobile services switching centers MSC, other network elements (which in the GSM include the serving GPRS support node, SGSN, and gateway GPRS support node, GGSN, for example), and the associated transmission systems. The radio access networks are located between the core network and mobile stations. A radio access network comprises base transceiver stations BTS and a radio network controller RNC. Each base station BS has a fixed connection to the radio network controller RNC. The radio network controllers in turn have fixed connections to at least one core network node. Between the mobile stations MS and core networks CN there may operate one or more radio access networks, and through a given radio access network a mobile station MS can be connected with several core networks CN.
FIG. 2 shows the coverage areas of services offered by a cellular system. Area 20 shows the coverage of basic GSM services, and areas 21, 28 and 29 are GPRS coverage areas within area 20. Typically, area 21 may be the center of a city, area 28 may be e.g. an airport, and area 29 may be a research and development or other office complex where GPRS service is needed.
FIG. 3 shows in more detail the GPRS base stations 23, 24, 25 and 26 and their coverage areas 23a, 24a, 25a and 26a in a city center 21. FIG. 3 does not show the basic-service GSM base stations the coverage areas of which are within area 21. FIG. 3 shows that even an area that offers GPRS services may have locations, here location 27, which are not within the coverage area of any GPRS base station. In such a location a mobile station has to connect to a base station which only offers the basic GSM service. In addition, it can be seen from FIG. 3 that when a mobile station is moving in an area where the base station coverage areas are small, the serving base station has to be changed at short intervals.
A mobile station in a cellular radio system always tries to choose a base station coverage area to camp on such that the quality of the radio connection is adequate. Traditionally, the cell selection has been based on the measurement of the received radio signal level either at the mobile station or at the base station For example, in the GSM system each base station transmits a signal on a so-called broadcast control channel BCCH which has different frequencies at adjacent base stations. Mobile stations measure the strengths of the BCCH signals they receive and decide on the basis of the measurements which cell is the most advantageous from the radio connection quality perspective. Base stations also inform mobile stations about the BCCH frequencies used in the neighbouring cells so that the mobile stations will know what frequencies they have to listen to in order to find the BCCH transmissions of the neighbouring cells. In each cell the BCCH channel transmission also includes information about how the mobile stations can make so-called random access requests in that particular cell in order to set up a connection.
FIG. 4 shows the GSM broadcast frequency channels in a so-called 51 multiframe. In addition to the BCCH, there are included the frequency correction channel FCCH, synchronization channel SCH, and the common control channel CCCH.
FIGS. 5a, 5b and 5c depict the GPRS 51 and 52 multiframe structures. In the GPRS broadcast channels the GSM BCCH and CCCH channels are replaced by their GPRS counterparts, namely the PBCCH and PCCCH channels. In the 51 multiframe structure, FIG. 5a, the FCCH and SCH channels are illustrated, even though they are not specified in the current GPRS specification. In the GPRS 52 multiframe structure no FCCH or SCH channels not shown, due to the fact that they are not specified in the 52 multiframe structure of the current specification. In the 52 multiframe structure shown in FIG. 5b there is PBCCH (PSI) only in one radio block per multiframe, while in the 52 multiframe structure shown in FIG. 5c there is PBCCH (PSI) information in four radio blocks per multiframe. Even though 51 multiframe structure is used as an example in this text, the ideas illustrated here are also valid for the 52 multiframe structures. This is because of the fact that there are no essential differences between the two frame structures In the GPRS system, the list of the base stations"" neighbour cells is sent in a so-called Packet Information type 3 (PSI3) and Packet Information type 3bis (PSI3bis) messages transmitted on the PBCCH channel.
In the GSM system, each base station includes in its BCCH signal the data required for connection requests. In addition, each base station may regularly transmit at the ECCH frequency so-called system information (SI) messages. These messages contain information about the ability of the base station to provide advanced services, especially GPRS network service.
If a mobile station camped on a cell that offers basic GSM service needs to change to another cell, it listens to the neighbouring cells"" BCCH channels and receives SI messages from these. On behalf of this information the mobile station can determine whether a cell supports GPRS service or not, and if it does, the mobile station can start operating on the GPRS channel immediately. For example, an idling mobile station can at regular intervals update its information about the neighbouring cells"" characteristics by receiving SI messages so that the information can be quickly retrieved from the mobile station""s memory at the moment of possible cell reselection.
FIG. 6 shows the prior-art process that takes place when a mobile station already in the GPRS service needs to change serving cell. The mobile station has already received a neighbour cell list from the serving base station. This list is used in order to continuously examine whether or not there is a need to change the serving cell, phase 60. If the mobile station finds that cell reselection is necessary, it tunes to the neighbour cell and starts receiving the system information of the BCCH of the chosen cell, phase 61. On the basis of these data the mobile station makes a preliminary selection of the cell and examines that the cell can offer a minimum of necessary service, phase 62. When the mobile station has chosen a suitable cell to get service from, the mobile station starts receiving the necessary SI or PSI from this cell in order to obtain GPRS service, phase 63. The chosen cell may or may not offer GPRS service.
A disadvantage of the method described above is that it is not sensible, as regards power consumption and communications resources, for the mobile station, while camped on a GPRS cell, to receive basic-GSM SI messages, but in practice the mobile station starts receiving SI messages only after the cell reselection procedure has started. Thus the mobile station does not keep an up-to-date list of the neighbour cell characteristics. The arrangement described above puts a load on the mobile station connected with the GPRS service because the mobile station camped on a cell first has to read from the signal sent by the base station of the cell a list of the other BCCH frequencies and then receive, demodulate and decode a short period of the BCCH transmission arriving at each of the frequencies mentioned in the list in order to find out whether the neighbour cells can provide GPRS network service. It may be that a major part of the neighbour cells do not support the GPRS, in which case the mobile station only wastes time and electric power in receiving, demodulating and decoding the BCCH channels of such cells. The reception of said GSM SI messages may take nearly 10 seconds, which usually is too long a delay in cell reselection, especially if the mobile station is moving fast. Consequently, the mobile station uses more power, data communications slows down and the connection with the base station may be temporarily cut off.
An object of this invention is to provide a method and arrangement in which the use of communications resources to find a new suitable cell is more efficient than in prior-art methods and arrangements.
An essential idea in the invention is that the serving base station sends to the mobile station the information about whether the base station in a neighbour cell provides a given service. This information is preferably sent in a signalling message on a channel of a given service, such as GPRS, to which the mobile station is connected. The cell reselection is then preferably performed directly at the level of the determined service.
The xe2x80x9cdetermined servicexe2x80x9d means here most preferably a service other than the lowest, basic service level of the communication system.
For example, when the mobile station is operating on GPRS packet channels, the information about whether a neighbour cell""s base station is offering GPRS or not, is advantageously sent in a PSI3 message on a GPRS packet channel. At its simplest, the invention can be realized in the GPRS system by adding to the packet channel""s PSI3 message a one-bit information element for each neighbour cell which indicates whether the neighbour cell in question supports the GPRS service. If the mobile station is not connected to a special service such as the GPRS, the information can be sent in the basic system""s information message, such as SI3.
The invention makes it possible for a mobile station connected to a given service to perform a direct change between the serving base station and another base station that offers the same service, without having to resort to lower-level system signalling. So, a mobile station connected to GPRS, for example, need not receive system information messages transmitted by the neighbour cells on the basic GSM channels in conjunction with cell reselection but it can select the new GPRS-supporting cell directly at GPRS level.
Cell reselection in mobile stations can be made more efficient than in prior-art systems if the mobile stations have prior knowledge of the service levels offered by the base stations. In the arrangement according to the invention a base station sends information about the service levels of the neighbouring base stations in signalling messages of the current service so that in order to find a new cell the mobile station receives, demodulates and decodes only the transmissions of those neighbouring base stations which transmit a strong enough signal and which according to the information sent by the current base station can offer the current service level. The mobile station may maintain various lists of such neighbouring base stations into whose cells it may move in the near future.
So, the invention helps adjust cell reselection such that the load on the mobile station and radio signalling, caused by measurement of possible new cells and reception of messages, is as small as possible.
In accordance with the invention a mobile station can as quickly as possible select from among available cells a new cell which best meets the communications needs of the mobile station. Cell reselection may be performed while the mobile station is in the so-called idle state, in which there is no active data connection between the mobile station and a base station, or it may be performed during an active data connection.
The method according to the invention for selecting a new serving cell in a cellular radio system where a determined service level is required in the new cell, the method including a phase of transmitting information about neighbouring cells from a base station of a cellular radio system to a mobile station, wherein the base station generates a message containing information about the neighbouring cells and sends the message to the mobile station, is characterized in that the base station includes in said message information about whether the neighbour cell mentioned in the message offers the determined service to mobile stations and the cell reselection is performed directly at the level of the determined service. The method according to the invention for realizing cell reselection in a cellular radio system which comprises base stations with associated cells as well as mobile stations, is characterized in that it comprises phases in which
a) a mobile station links up with a service offered by the system,
b) a base station sends to the mobile station a signalling message in accordance with a determined service, containing information about neighbour cells located near to the base station as well as information about whether said neighbour cells provide a determined service,
c) on the basis of said message the mobile station preliminarily selects a group of new cells offering a determined service,
d) the mobile station performs measurements among the preliminarily selected cells in order to find a suitable new cell, whereby
if the measurements indicate that among the preliminarily selected cells there is at least one suitable new cell, the mobile station selects a new cell from among the preliminarily selected cells, and
if the measurements indicate that among the preliminarily selected cells there is no suitable new cell, the mobile station selects a new cell from among cells other than the preliminarily selected cells, and
e) the cell reselection is performed directly at the level of the determined service.
The invention is also directed to a method for transmitting information about neighbouring cells from a base station of a cellular radio system to a mobile station, wherein the base station generates a message containing information about the neighbouring cells and sends the message to the mobile station, and which is characterized in that the base station includes in said message information about the service level offered to mobile stations by the neighbour cell mentioned in the message and that said signalling message includes at least one of the following pieces of information:
whether or not the neighbour cell is in blocking state,
length of the multiframe used by the neighbour cell,
location of control channel synchronization information.
The invention is also directed to a base station and mobile station of a cellular radio system.
The base station according to the invention, comprising means for offering a determined service and means for generating signalling messages and means for transmitting them to mobile stations, is characterized in that
it contains information about a group of neighbour cells located in the vicinity of the base station, including information about whether a neighbour cell provides said determined service,
it is equipped so as to include said information in a signalling message, and
it is equipped so as to the cell selection is perform the cell reselection directly at the level of the determined service.
The cellular mobile station according to the invention, equipped with means for connecting to a determined service, and comprising means for receiving signalling messages from base stations as well as means for performing cell-specific measurements to find a suitable cell for the purpose of cell reselection, is characterized in that it is equipped so as to indicate information about the service level of a neighbour cell based on signalling messages transmitted by the current serving base station, and to perform cell reselection on the basis of said information directly at the level of the determined service.
The invention is further directed to a cellular radio system, which comprises base stations and, in association with them, cells, as well as mobile stations and in which
the base stations are equipped so as to generate messages and send them to mobile stations in the form of signalling messages, and
the mobile stations are equipped so as to operate at a determined service level and receive signalling messages from base stations, and which is characterized in that it contains information accessible by a base station about a group of neighbour cells in the vicinity of the cell of the base station, including information about the service level offered in and by said neighbour cells to mobile stations, the system in question being arranged so as to transmit said information from the base station to a mobile station in a signalling message for the purpose of selecting a new serving cell to perform the cell reselection directly at the level of the determined service.
Preferred embodiments of the invention are described in the dependent claims.